The thermodynamic, structural and mechanical properties of polyolefins depend strongly on the type, content and distribution of defects generated during synthesis. In the last two decades, the spectrum of new polyolefin materials has undergone an exponential expansion due to the development of metal coordination catalysts with properties changing from those of highly-crystalline thermoplastics to plastomers and elastomers. The materials that have been developed, which range from statistical random copolymers to stereo-blocky elastomers, exemplify the ability of new synthetic strategies to tailor structures and properties of polyolefins by a suitable choice of the coordination catalyst precursor. The availability of such a wide spectrum of polyolefins has led to significant improvements in the approaches taken to characterize their molecular microstructure, which remains a main determinant of their physical properties. Ethylene α-olefin copolymers, polypropylene homopolymer and propylene α-olefin copolymers synthesized with single-site metallocene catalysts (early metal catalyst) serve as models to predict properties of polyolefins with randomly distributed defects.
Polyolefins (isotactic or syndiotactic) are highly crystalline polymers. Polyolefins result from the polymerization of an olefin (a linear or branched hydrocarbon with at least a double bond). Typical olefins are ethene, propene, 1-butene, 3-methyl pentene, 1-hexene . . . etc. For many applications that require increasing tear and impact properties or those with the need of more transparent, more flexible and ductile materials, defects or 1-alkene comonomers are added to the polyolefin backbone to decrease crystallinity.
A new family of polypropylenes (PPs), a type of polyolefins, has been synthesized by living polymerization with late metal catalysts (see for example Cherian, A. E.; Rose, J. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2005, 127, 13770-13771). These polymers are structurally distinct from PPs synthesized with early metal catalysts. Late metal catalysts enable chain-walking events; early metal catalysts do not. Specifically for iPP, the chain-walking mechanism adds (3,1) enchainments and often results in unique multi-monomer, bulky defect microstructures. A living chiral α-diimine Ni(II) catalyst can form isotactic PPs (iPPs) and allow controlled chain walking when activated with methylalumoxane (MAO) in the presence of propylene. An amorphous, regioirregular polymer is produced at high reaction temperatures (Trxn=0° C.) while an isotactic, regioregular polymer is obtained at low reaction temperatures (Trxn=−60° C.). Later modifications to the α-diimine Ni(II) catalyst have yielded isotactic polyolefins at an increased rate of polymerization (see for example Rose, J. M.; Deplace, F.; Lynd, N. A.; Wang, Z.; Hotta, A.; Lobkovsky, E. B.; Kramer, E. J.; Coates, G. W. Macromolecules 2008, 41, 9548-9555).
Enchainment via chain walking is a distinctive feature of polyolefins synthesized with the late metal catalysts resulting in extra CH2 in the iPP backbone compared to defects generated by inversion or 1-alkene comonomers. The result is a chain straightening due to the extended length of the defect in the backbone, as shown schematically in FIG. 1. Crystalline polyolefins with defects generated via chain walking have unique properties, such as reduced crystallinity (see for example C. Ruiz-Orta, J. P. Fernandez-Blazquez, A. M. Anderson-Wile, G. W. Coates, R. G. Alamo Macromolecules 2011, 44, 3436-3451.
At present, industry reduces the crystallinity of conventional polyolefins by polymer blending or copolymerization with ethylene, 1-butene, 1-hexene or 1-octene. The novel polyolefins with chain walking defects, when used in industry, will reduce the cost of materials production by eliminating polymer blending or addition and control of a comonomer. Polyolefins with chain-walking defects that lead to chain straightening display reduced crystallinity relative to conventional polyolefins with the same number of defects. The novel polyolefins of the present invention can substitute for present-art polyolefins in a variety of applications, for example, thin films, fibers and molded parts, or any other application that require a lower crystallinity than for the homopolymer or copolymer free of chain-walking defects. The reduced crystallinity makes the polymeric materials with better processability due to their lower melting temperatures, more flexible and more transparent.